drm/i915/lrc: Remove superfluous WARN_ON

Remove the WARN_ON(ce->state) inside the static function only called
when ce->state == NULL and downgrade the w/a batch setup warning into a
developer only mode (GEM_WARN_ON).

v2: Move the deferred alloc guard into the callee, eliminating the need
for the WARN_ON:
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-1 (-1)
Function                                     old     new   delta
execlists_context_pin                       1819    1818      -1

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180126121846.12007-1-chris@chris-wilson.co.uk